Optical fibers typically contain a glass core and at least two coatings, e.g., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled; but it also protects the glass surface from water adsorption, which can promote crack growth and increase static fatigue that result in failure. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the secondary coating. Before curing, the secondary coating composition should have a suitable viscosity and be capable of curing quickly to enable processing of the optical fiber. After curing, the secondary coating should have the following characteristics: sufficient stiffness to protect the encapsulated glass fiber yet enough flexibility for handling (i.e., modulus), low water absorption, low tackiness to enable handling of the optical fiber, chemical resistance, and sufficient adhesion to the primary coating.
Certain characteristics are desirable for the primary coating. Before curing, the primary coating composition should also have suitable viscosity and be capable of curing quickly to enable processing of the optical fiber. After curing, the primary coating must have a modulus that is sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained throughout the fiber's lifetime. Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) that is lower than the foreseeable lowest use temperature, which enables the primary coating to remain elastic throughout the temperature range of use. Finally, it is important for the primary coating to have good glass adhesion properties, yet be mechanically removable from an individual fiber or from a ribbon with reasonable force while leaving insubstantial residue (preferably none).
These requirements place conflicting constraints on the coatings, and especially on the primary coating. Ribbon stripping performance and mechanical damage to the primary are worse when the primary coating is soft and thick, for example, while microbending resistance improves under the same conditions. Protection against static fatigue is also generally worse when the coating is very soft.
To date, manufacturers have offered only coatings that are a compromise between these properties. In response to requirements for more microbend-resistant coatings for fibers in high-density or very small cables, commercial coatings are softer than they were 10 years ago, but throughout, the basic two-layer structure of the fiber coating has not changed. Cablers are continuing to press for further improvements, however, and the two-layer composite may no longer be adequate. It would be desirable, therefore, to develop an optical fiber coating system that improves microbend performance and failure rates due to fatigue over that achieved by conventional two-coating systems, while also maintaining or improving coating stripability.
The present invention is directed to overcoming these deficiencies in the art.